My external laptop drive (it has both USB and FireWire ports) actually had a similar problem.
On my Windows-based notebooks, the drive pulled sufficient power through the USB port to fire up and run.
On my Powerbook (Rev D. 15" 1.67 blah blah), it drew *almost enough*. The LED came on, and the drive switched on - but the HDD just sat there, whirring and not completely spinning up. Didn't show up in profiler, didn't read in the USB tree, and didn't mount.
I thought "Well, that's wierd" and plugged in the USB-Secondary Power cord that came with my enclosure. Bam! Worked like a charm.
So I thought "Hm, that's a little annoying. What about FireWire?"
The drive pulled sufficient power through the PB's Firewire 400 connector, evidenced by the drive powering up and correctly mounting.
By this I have to deduce that the PowerBook G4 doesn't supply the same power over its USB2.0 ports that some other machines do (this was both on battery and AC, by the by). As a result, external hard disks - which require a lot of power - may not work unless powered either by its own power cord, Firewire, or the second USB port on the Powerbook.
Your mileage may, of course, vary, but this was my experience.